Biologically inert, biocompatible-polymer conjugates

ABSTRACT

Pharmaceutically acceptable, non-immunogenic compositions are formed by covalently binding biologically inactive, natural, biocompatible polymer to pharmaceutically pure, synthetic, hydrophilic polymers via specific types of chemical bonds to provide biocompatible conjugates. The synthetic hydrophilic polymer may be polyethylene glycol and derivatives thereof having a weight average molecular weight over a range of from about 100 to about 20,000. The compositions may include other components such as liquid, pharmaceutically acceptable, carriers to form injectable formulations, and/or biologically active proteins such as growth factors. The conjugates of the invention generally contain large amounts of water when formed. The conjugates can be dehydrated to form a relatively solid object. The dehydrated, solid object can be ground into particles which can be suspended in a non-aqueous fluid such as an oil and injected into a living (preferably human) being for the purpose of providing soft tissue augmentation. Once in place, the particles rehydrate and expand in size five fold or more.

CROSS-REFERENCES

This application is a continuation-in-part of copending U.S. application Ser. No. 07/433,441, filed Nov. 14, 1989 and now U.S. Pat. No. 5,162,430, which is a continuation-in-part of U.S. application Ser. No. 07/274,071, filed Nov. 21, 1988 and now abandoned, both of which are incorporated herein by reference in full and to which applications we claim priority under 35 USC ∫120.

DESCRIPTION

1. Technical Field

This invention relates to biocompatible conjugates and specifically to pharmaceutically acceptable, non-immunogenic, compositions comprising insoluble, biologically inert, biocompatible polymer which is preferably a naturally-occurring polymer or derivative thereof conjugated together via synthetic hydrophilic polymers such as polyethylene glycol (PEG).

2. Background of the Invention

Collagen is the major protein component of bone, cartilage, skin, and connective tissue in animals. Collagen in its native form is typically a rigid, rodshaped molecule approximately 300 nm long and 1.5 nm in diameter. It is composed of three collagen polypeptides which form a tight triple helix. The collagen polypeptides are characterized by a long midsection having the repeating sequence --Gly--X--Y--, where X and Y are often proline or hydroxyproline, bounded at each end by the "telopeptide" regions, which constitute less than about 5% of the molecule. The telopeptide regions of the collagen chains are typically responsible for the crosslinking between chains, and for the immunogenicity of the protein. Collagen occurs in several "types", having differing physical properties. The most abundant types are Types I-III.

Collagen is typically isolated from natural sources, such as bovine hide, cartilage, or bones. Bones are usually dried, defatted, crushed, and demineralized to extract collagen, while hide and cartilage are usually minced and digested with proteolytic enzymes (other than collagenase). As collagen is resistant to most proteolytic enzymes, this procedure conveniently serves to remove most of the contaminating protein found with collagen.

In our earlier copending U.S. application Ser. No. 07/433,441, we described chemical conjugates whereby various forms of collagen were conjugated using synthetic hydrophilic polymers such as polyethylene glycol. Such conjugates are useful for a variety of applications such as soft tissue augmentation and the formation of implants useful in bone repair. We have now found that it is possible to form such conjugates with biomaterial other than collagen. Specifically, synthetic hydrophilic polymers are used to link insoluble biocompatible, biologically inert polymers (preferably naturally-occurring polymers) other than collagen. Polyethylene glycol is the preferred linker. Accordingly, we now describe a variety of biocompatible polymer conjugates and their methods of synthesis, which conjugates can be used in a manner similar to the collagen-polymer conjugates described in our earlier, above-referenced patent application.

SUMMARY OF THE INVENTION

Biocompatible pharmaceutically acceptable, nonimmunogenic compositions are formed by covalently binding insoluble, naturally-occurring, biologically inert polymers using pharmaceutically pure, synthetic, hydrophilic polymers such as polyethylene glycol (PEG) to provide biocompatible conjugates.

The insoluble, biologically inert polymers. (preferably naturally-occurring polymers and derivatives thereof) include polysaccharides such as hyaluronic acid, proteoglycans such as chondroitin sulfate-A (4-sulfate) chondroitin sulfate C (6-sulfate) and dermatan sulfate (chondroitin sulfate B) sulfate; dextrins such as cyclodextrin, hydroxylethyl cellulose, cellulose ether and starch; lipids (esters of fatty acids with trihydroxyl alcohol glycerol) such as triglyceride and phospholipids and synthetic polymers such as polyethylene, polyurethane, polylactic acid, polyglycolic acid and the like.

The synthetic hydrophilic polymer is preferably polyethylene glycol and derivatives thereof having a weight average molecular weight over a range of from about 100 to about 20,000 preferably about 1,500 to 20,000. The compositions may include other components such as liquid, pharmaceutically acceptable, carriers to form injectable formulations, and/or biologically active proteins such as cytokines. The biocompatible conjugates of the invention generally contain large amounts of water when formed. The conjugates can be dehydrated to form a relatively solid object. The dehydrated, solid object can be ground into particles which can be suspended in a non-aqueous fluid such as an oil and injected into a living being for the purpose of providing soft tissue augmentation. Once in place, the particles rehydrate and expand in size five fold or more.

The essence of the invention relates to the biocompatible conjugates which are applied and used in a variety of medical and pharmaceutical applications. Although all aspects of the invention generally relate to these conjugates, the invention can be categorized into the following six different embodiments: (1) The most basic embodiment includes the biocompatible conjugates and pharmaceutical compositions which are formulated using these conjugates, which compositions include pharmaceutically acceptable carriers in different types and amounts. (2) One of the most important uses for the conjugates and compositions of the invention is in methods of effecting soft tissue augmentation. The compositions are formulated in a flowable form and injected into patients, such as into facial areas, in order to provide for soft tissue augmentation. The method can be varied so that the reaction between the naturally-occurring polymer and the synthetic polymer occurs in situ. Furthermore, the conjugates can be dehydrated and then ground into particles, suspended in an inert nonaqueous carrier, and injected into a patient. After injection, the carrier will be removed by natural physiological conditions and the particles will rehydrate and swell to their original size. (3) All types of conjugates and conjugate compositions of the invention can be combined with cytokines. (4) The various conjugates and compositions of the invention can be further combined with particles and materials in order to increase the structural integrity of the compositions so that they can be used in the augmentation of hard tissue, such as bone and cartilage. (5) The conjugates and compositions containing the conjugates can be coated on to various medical devices, including catheters, and bone implants and platinum wires to treat aneurysms. (6) The conjugates can be formulated into various ophthalmic devices, such as lenticulas or corneal shields. The cytokines may be either simply admitted with the PEG-polymer conjugate or chemically conjugated to di- or multifunctional PEG-polymer (polymer-PEG-cytokine). In the case of an admixture, as the cytokines are not chemically bound to the PEG-polymer, the cytokines may diffuse out from the site of administration into the surrounding tissue, providing for sustained release and local therapeutic effects. In the case of the cytokine being actually chemically conjugated to the polymer conjugate, the cytokine retains its biological activity even while bound to the conjugate.

A primary object of the invention is to provide biocompatible conjugates formed by covalently binding polymers such as polyethylene glycol to non-immunogenic forms of naturally occurring, insoluble, biologically inert polymers such as hyaluronic acid and chondroitin sulfate.

Another object of the invention is to provide pharmaceutically acceptable, non-immunogenic compositions comprising pharmaceutically acceptable liquid carriers having the biocompatible conjugates therein.

Another object of the invention is to provide a method of tissue augmentation comprising forming the biocompatible conjugates, dehydrating the conjugates to form a solid, grinding the solid into particles, suspending the particles in a liquid non-aqueous carrier, and injecting the suspension into the site of augmentation, after which the particles will rehydrate and expand in size.

An important advantage of the present invention is that the biocompatible conjugates have a high degree of stability over long periods of time under physiological conditions.

A feature of the invention is that the conjugates can be formed using a range of different molecular weight polymers in order to adjust physical characteristics of the resulting composition.

Another advantage of the present invention is that the biocompatible conjugates have superior handling characteristics as compared with conventional collagen compositions.

Another advantage of the present invention is that the biocompatible conjugate compositions generate a decreased immune reaction as compared with conventional collagen compositions.

Other advantages and features of the present invention is that the biocompatible conjugate compositions have improved moldability, malleability, and elasticity as compared with conventional collagen compositions.

Other features of the present invention include the ability to formulate the compositions and conjugates in combination with pharmaceutically active proteins such as cytokines (growth factors) in order to improve the activity and available half-life of such cytokines (growth factors) under physiological conditions.

Another feature of the present invention is that the natural polymer or derivative thereof may be a bound to the polymer by means of a variety of types of covalent linkages including ester and ether linkages.

Another advantage of the present invention is an ether linkage may be used to form the covalent bond to create the conjugate and this bond is resistant to breakage due to hydrolysis.

These and other objects, advantages and features of the present invention will become apparent to those persons skilled in the art upon reading the details of the structure, synthesis and usage of the biocompatible conjugates as more fully set forth below, reference being made to the specific examples and formulations forming a part hereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Before the present biocompatible conjugates and processes for making and using such are described, it is to be understood that this invention is not limited to the particular conjugates or processes and methods described as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting as the scope of the present invention will be limited only by the appended claims.

It must be noted that, as used in this specification and the appended claims, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a naturally-occurring polymer" includes mixtures of such polymers reference to "an attaching group or a linking group" includes one or more different types of groups known by those skilled in the art or capable of forming a covalent bond and reference to "the synthetic polymer" includes mixtures of different types of polymers such as PEG and so forth.

Unless defined otherwise all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein may be useful in the practice or testing of the present invention, preferred methods and materials are described below. All publications mentioned herein are incorporated herein by reference. Further, specific terminology of particular importance to the description of the present invention is defined below.

A. DEFINITIONS

The term "collagen" as used herein refers to all forms of collagen, including those which have been processed or modified. To form the biocompatible conjugates of the invention, the collagen may be of human or animal origin and may be produced by recombinant techniques.

The terms "naturally-occurring polymers" and "natural polymer" as used herein refers to biologically inert, insoluble, naturally-occurring, biocompatible polymers and derivatives thereof including polysaccharides such as hyaluronic acid; proteoglycans such as chondroitin sulfate A (4-sulfate), chondroitin sulfate C (6-sulfate), and dermatan sulfate (chondroitin sulfate B); dextrans such as cyclodextrin, hydroxylethyl cellulose, cellulose ether and starch; lipids (esters of fatty acids with trihydroxyl alcohol glycerol) such as triglyceride and phospholipids.

The terms "biologically inert polymers", "biocompatible polymers" and "biologically inert biocompatible polymers" are used interchangeably herein. The terms refer to biologically inert, insoluble, biocompatible polymers and their derivatives which can be covalently bound to the synthetic hydrophilic polymer in order to form conjugates of the invention. These terms encompass the above-defined "natural polymers" but further include synthetic polymers having similar characteristics to the natural polymers in that they are biologically inert, insoluble, nontoxic and do not generate any immune reaction when incorporated into a living being such as a human.

The term "synthetic hydrophilic polymer" as used herein refers to a synthetic polymer having an average molecular weight and composition which renders the polymer essentially water-soluble. Preferred polymers are highly pure or are purified to a highly pure state such that the polymer is or is treated to become pharmaceutically pure. Most hydrophilic polymers can be rendered water-soluble by incorporating a sufficient number of oxygen (or less frequently nitrogen) atoms available for forming hydrogen bonds in aqueous solutions. Preferred polymers are hydrophilic but not necessarily soluble. Hydrophilic polymers used herein include polyethylene glycol (PEG), polyoxyethylene, polymethylene glycol, polytrimethylene glycols, polyvinyl-pyrrolidones, and derivatives thereof with PEG being particularly preferred. The polymers can be linear or multiply branched, and will not be substantially crosslinked. Other suitable polymers include polyoxyethylenepolyoxypropylene block polymers and copolymers. Polyoxyethylene-polyoxypropylene block polymers having an ethylene diamine nucleus (and thus having four ends) are also available and may be used in the practice of the invention. Naturally occurring polymers such as proteins, starch, cellulose, heparin, and the like are expressly excluded from the scope of this definition. All suitable polymers will be non-toxic, non-inflammatory, and non-immunogenic when administered subcutaneously, and will preferably be essentially nondegradable in vivo over a period of at least several months. The hydrophilic polymer may increase the hydrophilicity of the collagen, but does not render it water-soluble. Presently preferred hydrophilic polymers are mono-, di-, and multifunctional polyethylene glycols (PEG). Monofunctional PEG has only one reactive hydroxy group, while difunctional PEG has reactive groups at each end. Monofunctional PEG preferably has an average molecular weight between about 100 and about 15,000, more preferably between about 200 and about 8,000, and most preferably about 4,000. Difunctional PEG preferably has an average molecular weight of about 400 to about 40,000, more preferably about 3,000 to about 10,000. Multifunctional PEG preferably has an averagemolecular weight between about 3,000 and 100,000.

PEG can be rendered monofunctional, by forming an alkylene ether group at one end. The alkylene-ether group may be any suitable alkoxy radical having 1-6 carbon atoms, for example, methoxy, ethoxy, propoxy, 2-propoxy, butoxy, hexyloxy, and the like. Methoxy is presently preferred. Difunctional PEG is provided by allowing a reactive hydroxy group at each end of the linear molecule. The reactive groups are preferably at the ends of the polymer, but may be provided along the length thereof. Multifunctional molecules are capable of crosslinking the compositions of the invention, and may be used to attach cytokines to collagen.

The term "chemically conjugated" as used herein means attached through a covalent chemical bond. In the practice of the invention, a synthetic hydrophilic polymer and biologically inert, biocompatible polymer may be chemically conjugated by using a linking radical, so that the synthetic polymer and natural polymer are each bound to the radical, but not directly to each other. The term "biocompatible conjugate" refers to a biologically inert, biocompatible polymer chemically conjugated to a synthetic hydrophilic polymer, within the meaning of this invention. Thus, "biocompatible polymer/PEG" (or "PEG/biocompatible polymer") denotes a composition of the invention wherein a biocompatible polymer is chemically conjugated to PEG. "Biocompatible polymer/PEG" refers to a biologically inert, biocompatible polymer of the invention chemically conjugated to difunctional PEG, wherein the biocompatible polymer molecules are typically crosslinked. The synthetic polymer may be "chemically conjugated" to the biocompatible polymer by means of a number of different types of chemical linkages. For example, the conjugation can be via an ester or a urethane linkage, but is more preferably by means of an ether linkage. An ether linkage is preferred in that it can be formed without the use of toxic chemicals and is not readily susceptible to hydrolysis in vivo.

Those of ordinary skill in the art will appreciate that synthetic polymers such as polyethylene glycol cannot be prepared practically to have exact molecular weights, and that the term "molecular weight" as used herein refers to the average molecular weight of a number of molecules in any given sample, as commonly used in the art. Thus, a sample of PEG 2,000 might contain a statistical mixture of polymer molecules ranging in weight from, for example, 1,500 to 2,500 daltons with one molecule differing slightly from the next over a range. Specification of a range of molecular weight indicates that the average molecular weight may be any value between the limits specified, and may include molecules outside those limits. Thus, a molecular weight range of about 800 to about 20,000 indicates an average molecular weight of at least about 800, ranging up to about 20 kDa.

The term "available lysine residue" as used herein refers to lysine side chains exposed on the outer surface of natural polymer molecules, which are positioned in a manner to allow reaction with activated PEG. The number of available lysine residues may be determined by reaction with sodium 2,4,6-trinitrobenzenesulfonate (TNBS).

The terms "treat" and "treatment" as used herein refer to augmentation, repair, prevention, or alleviation of defects, particularly defects due to loss or absence of soft tissue or soft tissue support, or to loss or absence of bone. Additionally, "treat" and "treatment" also refer to the prevention, maintenance, or alleviation of disorders or disease using a biologically active protein coupled to a conjugate-containing composition of the invention. Accordingly, treatment of soft tissue includes augmentation of soft tissue, for example, implantation of conjugates of the invention to restore normal or desirable dermal contours, as in the removal of dermal creases or furrows, or as in the replacement of subcutaneous fat in maxillary areas where the fat is lost due to aging or in the augmentation of submucosal tissue, such as the urinary or lower esophageal sphincters. Treatment of bone and cartilage includes the use of conjugates, and particularly natural polymer/PEG in combination with suitable particulate materials, to replace or repair bone tissue, for example, in the treatment of bone nonunions or fractures. Treatment of bone also includes use of conjugate-containing compositions, with or without additional bone growth factors. Compositions comprising conjugates with ceramic particles, preferably hydroxyapatite and/or tricalcium phosphate, are particularly useful for the repair of stress-bearing bone due to its high tensile strength.

The term "cytokine" is used to describe biologically active molecules which aid in healing or regrowth of normal tissue including growth factors and active peptides. The function of cytokines is two-fold: 1) they can incite local cells to produce new collagen or tissue, or 2) they can attract cells to the site in need of correction. As such, cytokines serve to encourage "biological anchoring" of the biocompatible implant within the host tissue. As previously described, the cytokines can either be admixed with the biocompatible conjugate or chemically coupled to the conjugate. For example, one may incorporate cytokines such as epidermal growth factor (EGF), transforming growth factor (TGF) alpha, TGF-β (including any combination of TGF-βs), TGF-β1, TGF-β2, platelet derived growth factor (PDGF-AA, PDGF-AB, PDGF-BB), acidic fibroblast growth factor (FGF), basic FGF, connective tissue activating peptides (CTAP), β-thromboglobulin, insulin-like growth factors, tumor necrosis factors (TNF), interleukins, colony stimulating factors (CSFs), erythropoietin (EPO), nerve growth factor (NGF), interferons (IFN), bone morphagenic protein (BMP), osteogenic factors, and the like. Incorporation of such cytokines, and appropriate combinations of cytokines, can facilitate the regrowth and remodeling of the implant into normal bone tissue, or may be used in the treatment of wounds. Further, one may chemically link the cytokines to the conjugates by employing a suitable amount of multifunctional polymer molecules during synthesis. The factors may then be attached to the free polymer ends by the same method used to attach PEG to the natural polymer, or by any other suitable method. By tethering factor molecules to the implant, the effective amount of factor is substantially reduced. Dried conjugate compositions having sponge-like characteristics may be prepared as wound dressings, or when incorporated with cytokines, they serve as effective controlled release drug delivery matrices. By varying the type of chemical linkage between the biocompatible polymer and the synthetic polymer, it is possible to vary the effect with respect to the release of the growth factor or other pharmaceutically active protein. For example, when an "ester" linkage is used, the linkage is more easily broken under physiological conditions, allowing for sustained release of the growth factor from the matrix. However, when an "ether" linkage is used, the bonds are not easily broken and the cytokine will remain in place for longer periods of time with its active sites exposed, providing a biological effect on the natural substrate for the active site of the protein. It is possible to include a mixture of conjugates with different linkages to obtain variations in the effect with respect to the release of the cytokine, i.e., the sustained release effect can be modified to obtain the desired rate of release.

The term "effective amount" refers to the amount of composition required in order to obtain the effect desired. Thus, a "tissue growth-promoting amount" of a composition containing a cytokine refers to the amount of cytokine needed in order to stimulate tissue growth to a detectable degree. Tissue, in this context, includes connective tissue, bone, cartilage, epidermis and dermis, blood, and other tissues. The actual amount which is determined to be an effective amount will vary depending on factors such as the size, condition, sex and age of the patient and can be more readily determined by the caregiver.

The term "sufficient amount" as used herein is applied to the amount of carrier used in combination with the conjugates of the invention. A sufficient amount is that amount which, when mixed with the conjugate, renders it in the physical form desired, for example, injectable solution, injectable suspension, plastic or malleable implant, rigid stress-bearing implant, and so forth. Injectable formulations generally include an amount of fluid carrier sufficient to render the composition smoothly injectable, whereas malleable implants have substantially less carrier and have a clay-like consistency. Rigid stress-bearing implants may include no carrier at all and have a high degree of structural integrity. The amount of the carrier can be varied and adjusted depending on the particular conjugate used and the end result desired. Such adjustments will be apparent to those skilled in the art upon reading this disclosure.

The term "suitable particulate material" as used herein refers to a particulate material which is substantially insoluble in water, non-immunogenic, biocompatible, and immiscible with collagen-polymer conjugate. The particles of material may be fibrillar, or may range in size from about 20 to 250 μm in diameter and be bead-like or irregular in shape. Exemplary particulate materials include without limitation fibrillar crosslinked collagen, gelatin beads, crosslinked collagen-dPEG particles, polytetrafluoroethylene beads, silicone rubber beads, hydrogel beads, silicon carbide beads, and glass beads. Preferred particulate materials are calcium phosphates, most preferably hydroxyapatite and/or tricalcium phosphate.

The term "solid implant" refers to any solid object which is designed for insertion and use within the body, and includes bone and cartilage implants (e.g., artificial joints, retaining pins, cranial plates, and the like, of metal, plastic and/or other materials), breast implants (e.g., silicone gel envelopes, foam forms, and the like), catheters and cannulas intended for long-term use (beyond about three days), artificial organs and vessels (e.g., artificial hearts, pancreases, kidneys, blood vessels, and the like), drug delivery devices (including monolithic implants, pumps and controlled release devices such as Alzet® minipumps, steroid pellets for anabolic growth or contraception, and the like), sutures for dermal or internal use, periodontal membranes, lenticulas, corneal shields, platinum wires for aneurysm treatment and the like.

The term "suitable fibrous material", as used herein, refers to a fibrous material which is substantially insoluble in water, non-immunogenic, biocompatible, and immiscible with the biocompatible conjugate of the invention. The fibrous material may comprise a variety of materials having these characteristics and are combined with compositions of the conjugate in order to form and/or provide structural integrity to various implants or devices used in connection with medical and pharmaceutical uses. For example, the conjugate compositions of the invention can be coated on the "suitable fibrous material" which can then be wrapped around a bone to provide structural integrity to the bone. Thus, the "suitable fibrous material" is useful in forming the "solid implants" of the invention.

The term "in situ" as used herein means at the site of administration. Thus, the injectable reaction mixture compositions are injected or otherwise applied to a site in need of augmentation, and allowed to crosslink at the site of injection. Suitable sites will generally be intradermal or subcutaneous regions for augmenting dermal support, at the site of bone fractures for wound healing and bone repair, and within sphincter tissue for sphincter augmentation (e.g., for restoration of continence).

The term "aqueous mixture" includes liquid solutions, suspensions, dispersions, colloids, and the like containing a natural polymer and water.

The term "NFC cartilage" as used herein refers to a composition of the invention which resembles cartilage in physical consistency. NFC cartilage is prepared from nonfibrillar collagen (e.g., collagen in solution) and is crosslinked with a hydrophilic polymer, especially using dPEG. As an artifact of the production process or by design, NFC cartilage may contain about 0-20% fibrillar collagen. NFC cartilage is generally prepared by adding dPEG in acidic solution to an acidic solution of collagen, and allowing conjugation to occur prior to neutralization. The term "NFC-FC cartilage" refers to a composition similar to NFC cartilage, wherein the percentage of fibrillar collagen is about 20-80%. NFC-FC cartilage is generally prepared by adding dPEG in a neutralizing buffer to an acidic solution of collagen. The neutralizing buffer causes collagen fibril formation during the conjugation process. Similarly, "FC cartilage" refers to a composition of the invention which is prepared from fibrillar collagen and a difunctional hydrophilic polymer. FC cartilage may generally be prepared using dPEG and fibrillar collagen in neutral solutions/suspensions.

B. GENERAL METHOD

B.1 Preparation

To form the conjugates of the invention a biologically inert, biocompatible polymer must be chemically bound to a synthetic hydrophilic polymer. This can be carried out in a variety of ways. In accordance with the preferred method, the synthetic hydrophilic polymer is activated and then reacted with the biocompatible polymer, preferably natural polymer. Alternatively, hydroxyl or amino groups present on the natural polymer can be activated and the activated groups will react with the polymer to form the conjugate. In accordance with a less preferred method, a linking group with activated hydroxyl or amino groups thereon can be combined with the synthetic polymer and biocompatible polymer in a manner so as to concurrently react the polymers to form the conjugate. Other methods of forming the conjugates will become apparent to those skilled in the art upon reading this disclosure. Since the conjugates of the invention are to be used in the human body, it is important that all of the components, including both polymers and linking group, if used, form a conjugate that is unlikely to be rejected by the patient. Accordingly, toxic and/or immunoreactive components are not preferred as starting materials. Some preferred starting materials and methods of forming conjugates are described further below.

Although different hydrophilic synthetic polymers can be used in connection with forming the conjugate, the polymer must be biocompatible, relatively insoluble, but hydrophilic and is preferably one or more forms of polyethylene glycol (PEG), due to its known biocompatibility. Various forms of PEG are extensively used in the modification of biologically active molecules because PEG can be formulated to have a wide range of solubilities and because it lacks toxicity, antigenicity, immunogenicity, and does not typically interfere with the enzymatic activities and/or conformations of peptides. Further, PEG is generally non-biodegradable and is easily excreted from most living organisms including humans.

The first step in forming the conjugates of the invention generally involves the functionalization of the PEG molecule. Various functionalized polyethylene glycols have been used effectively in fields such as protein modification (see Abuchowski et al., Enzymes as Druas, John Wiley & Sons: New York, N.Y. (1981) pp. 367-383; and Dreborg et al., Crit. Rev. Therap. Drug Carrier Syst. (1990) 6:315, both of which are incorporated herein by reference), peptide chemistry (see Mutter et al., The Peptides, Academic: New York, N.Y. 2:285-332; and Zalipsky et al., Int. J. Peptide Protein Res. (1987) 30:740, both of which are incorporated herein by reference), and the synthesis of polymeric drugs (see Zalipsky et al., Eur. Polym. J. (1983) 19:1177; and Ouchi et al., J. Macromol. Sci. -Chem. (1987) A24:1011, both of which are incorporated herein by reference). Various types of conjugates formed by the binding of polyethylene glycol with specific pharmaceutically active proteins have been disclosed and found to have useful medical applications in part due to the stability of such conjugates with respect to proteolytic digestion, reduced immunogenicity, and longer half-lives within living organisms.

One form of polyethylene glycol which has been found to be particularly useful is monomethoxypolyethylene glycol (mPEG), which can be activated by the addition of a compound such as cyanuric chloride, then coupled to a protein (see Abuchowski et al., J. Biol. Chem. (1977) 252:3578, which is incorporated herein by reference). Although such methods of activating polyethylene glycol can be used in connection with the present invention, they are not particularly desirable in that the cyanuric chloride is relatively toxic and must be completely removed from any resulting product in order to provide a pharmaceutically acceptable composition.

Activated forms of PEG, including activated forms of mPEG, can be made from reactants which can be purchased commercially. One form of activated PEG which has been found to be particularly useful in connection with the present invention is mPEG-succinate-N-hydroxysuccinimide ester (SS-PEG) (see Abuchowski et al., Cancer Biochem. Biphys. (1984) 7:175, which is Incorporated herein by reference). Activated forms of PEG such as SS-PEG react with the proteins under relatively mild conditions and produce conjugates without destroying the specific biological activity and specificity of the protein attached to the PEG. However, when such activated PEGs are reacted with proteins, they react and form linkages by means of ester bonds. Although ester linkages can be used in connection with the present invention, they are not particularly preferred in that they undergo hydrolysis when subjected to physiological conditions over extended periods of time (see Dreborg et al., Crit. Rev. Therap. Drug Carrier Syst. (1990) 6:315; and Ulbrich et al., J. Makromol. Chem. (1986) 187:1131, both of which are incorporated herein by reference).

It is possible to link PEG to proteins via urethane linkages, thereby providing a more stable attachment which is more resistant to hydrolytic digestion than the ester linkages (see zalipsky et al., Polymeric Drug and Drug Delivery Systems, Chapter 10, "Succinimidyl Carbonates of Polyethylene Glycol" (1991) incorporated herein by reference to disclose the chemistry involved in linking various forms of PEG to specific biologically active proteins). The stability of urethane linkages has been demonstrated under physiological conditions (see Veronese et al., Appl. Biochem. Biotechnol. (1985) 11:141; and Larwood et al., J. Labelled Compounds Radiopharm. (1984) 21:603, both of which are incorporated herein by reference). Another means of attaching the PEG to a protein can be by means of a carbamate linkage (see Beauchamp et al., Anal. BioChem. (1983) 131:25; and Berger et al., Blood (1988) 71:1641, both of which are incorporated herein by reference). The carbamate linkage is created by the use of carbonyldiimidazole-activated PEG. Although such linkages have advantages, the reactions are relatively slow and may take 2 to 3 days to complete.

The various means of activating PEG described above and publications (all of which are incorporated herein by reference) cited in connection with the activation means are described in connection with linking the PEG to specific biologically active proteins and not inert, biologically inactive, natural polymers. (See Polymeric Drug and Drug Delivery Systems, Chapter 10, "Succinimidyl Carbonates of Polyethylene Glycol" (1991), incorporated herein by reference to disclose the chemistry involved in linking various forms of PEG to specific biologically active proteins.) However, the present invention now discloses that such activated PEG compounds can be used in connection with the formation of inert, biocompatible conjugates. Such conjugates provide a range of improved, unexpected characteristics and as such can be used to form the various compositions of the present invention.

B.2 Specific Forms of Activated PEG

As indicated above, the conjugates of the present invention can be prepared by covalently binding a variety of different types of synthetic hydrophilic polymers to a natural polymer or other biocompatible synthetic polymer. However, in that the final product or conjugate obtained must have a number of characteristics such as being biocompatible and non-immunogenic, it has been found that it is useful to use polyethylene glycol as the synthetic hydrophilic polymer. The polyethylene glycol must be modified in order to provide activated groups on one or preferably both ends of the molecule so that covalent binding can occur between the PEG and the natural polymer such as collagen. Some specific functionalized forms of PEG are shown structurally below, as are the reaction products obtained by reacting these functionalized forms of PEG with collagen and other natural and/or biocompatible polymers.

1. PEG CONJUGATION OF COLLAGEN

The first functionalized PEG is difunctionalized PEG succinimidyl glutarate, referred to herein as (SG-PEG). The structural formula of this molecule and the reaction product obtained by reacting it with collagen is shown below: ##STR1##

Another difunctionally activated form of PEG is referred to as PEG succinimidyl (S-PEG). The structural formula for this compound and the reaction product obtained by reacting it with collagen is shown below. It should be noted that the methyl group is repeated twice. In a general structural formula for this compound, the 2 is replace with an "n" and in this embodiment, n=3, in that there are three repeating CH₂ groups on each side of the PEG. The above structure results in a conjugate which includes a "ether" linkage which is not subject to hydrolysis. This is distinct from the first conjugate shown above, wherein an ester linkage is provided. The ester linkage is subject to hydrolysis under physiological conditions. ##STR2##

Yet another derivatized form of polyethylene glycol, wherein n=2 is shown below, as is the conjugate formed by reacting the derivatized PEG with collagen. ##STR3## In another preferred embodiment of the invention similar to the immediate two above formulas, is provided when n=1. The structural formula and resulting conjugate are shown below. It is noted that the conjugate includes both an ether and a peptide linkage. These linkages are stable under physiological conditions. ##STR4## Yet another derivatized form of PEG is provided when n=0. The difunctionalized form is referred to as PEG succinimidyl carbonate (SC-PEG). The structural formula of this compound and the conjugate formed by reacting SC-PEG with collagen is shown below. Although this conjugate includes a urethane linkage, the conjugate has been found not to have a high degree of stability under physiological conditions. ##STR5##

All of the above derivatives involve the inclusion of the succinimidyl group. However, different activating groups can be attached to one or both ends of the PEG. For example, the PEG can be derivatized to form difunctional PEG propion aldehyde (A-PEG), which is shown below as is the conjugate formed by the reaction of A-PEG with collagen. ##STR6##

Yet another functionalized form of polyethylene glycol is difunctional PEG glycidyl ether (E-PEG), which is shown below, as is the conjugate formed by reacting such with collagen. ##STR7##

The conjugates formed using the functionalized forms of PEG vary depending on the functionalized form of PEG which is used in the reaction. Further, the final product can be varied with respect to its characteristics by changing the molecular weight of the PEG. In general, the stability of the conjugate is improved by eliminating any ester linkages between the PEG and the collagen and including ether and/or urethane linkages. In certain situations, it is desirable to include the weaker ester linkages so that the linkages are gradually broken by hydrolysis under physiological conditions, breaking apart the matrix and releasing a component held therein, such as a growth factor. By varying the chemical structure of the linkages, the rate of sustained release can be varied.

2. PEG CONJUGATION OF POLYSACCHARIDES

Hyaluronic acid comprises a polymer of the following repeating monomeric units. ##STR8##

Hyaluronic acid can be conjugated to PEG using a number of different methods. Preferred methods include modification of the carboxyl group and modification of the acyl group. ##STR9##

3. PEG CONJUGATION OF PROTEAGLYCANS

As shown below, chondroitin sulfate A, chondroitin sulfate C, and dermatan sulfate (chondroitin sulfate C) are of similar structure to hyaluronic acid and can be conjugated to PEG using the same methods described in the previous section. ##STR10##

4. PEG CONJUGATION OF POLYLACTIC ACID

Esters undergo nucleophilic substitution, which is typical of carboxylic acid derivatives. Attack occurs at the electron-deficient carbonyl carbon. These reactions are sometimes carried out in the presence of acid. In these acid-catalyzed reactions, H+ attaches itself to the oxygen of the carbonyl group, rendering the carbonyl carbon even more susceptible to nucleophilic attack. ##STR11##

5. PEG CONJUGATION OF POLYETHYLENE

A number of different activated PEGs, both difunctional and multifunctional, can be used to crosslink polyethylene.

a. Use of Difunctional or Multifunctional E-PEG:

Difunctional or multifunctional E-PEG can be used for crosslinking polyethylene due to the ease of opening the highly strained three-member ring. The bond angles of the ring, which average 60 degrees, are considerably smaller than the normal tetrahedral carbon angles of 109 degrees. Because polyethylene is not readily denatured at elevated temperatures (as are many natural polymers, such as collagen), E-PEG may be used to crosslink polyethylene at elevated temperatures. ##STR12##

The crosslinking reaction between the natural polymer and synthetic polymer may be performed in vitro, or a reaction mixture may be injected for crosslinking in situ. At sufficient density, crosslinked biocompatible conjugates resemble cartilage, and are useful as substitutes therefore, (e.g., cranial onlay, ear and nose reconstruction, and the like). Multifunctional polymers may also be used to crosslink natural polymers to other proteins, particularly cytokines, for compositions particularly suited for wound healing, osteogenesis, and immune modulation. Such tethering of growth factors to biocompatible polymers such as collagen molecules provides an effective slow-release drug delivery system.

Suitable collagens include all types of collagen and preferably non-immunoreactive, atelopeptide collagen, with types I, II and III being preferred. Collagens may be soluble (for example, commercially available Vitrogen® collagen-in-solution), and may have the telopeptide regions although such regions are generally omitted. Preferably, the collagen will be reconstituted fibrillar atelopeptide collagen, for example Zyderm® collagen implant (ZCI) or atelopeptide collagen in solution (CIS). Various forms of collagen are available commercially, or may be prepared by the processes described in, for example, U.S. Pat. Nos. 3,949,073; 4,488,911; 4,424,208; 4,582,640; 4,642,117; 4,557,764; and 4,689,399, all incorporated herein by reference. Non-fibrillar, atelopeptide, reconstituted collagen is preferred in order to form certain products.

The compositions of the invention comprise biocompatible polymer, preferably natural polymers chemically conjugated to a selected synthetic hydrophilic polymer or polymers. Collagen contains a number of available amino and hydroxy groups which may be used to bind the synthetic hydrophilic polymer. The polymer may be bound using a "linking group", as the native hydroxy or amino groups in collagen and in the polymer frequently require activation before they can be linked. For example, one may employ compounds such as dicarboxylic anhydrides (e.g., glutaric or succinic anhydride) to form a polymer derivative (e.g., succinate), which may then be activated by esterification with a convenient leaving group, for example, N-hydroxysuccinimide, N,N'-disuccinimidyl oxalate, N,N'-disuccinimidyl carbonate, and the like. See also Davis, U.S. Pat. No. 4,179,337 for additional linking groups. Presently preferred dicarboxylic anhydrides that are used to form polymer-glutarate compositions include glutaric anhydride, adipic anhydride, 1,8-naphthalene dicarboxylic anhydride, and 1,4,5,8-naphthalenetetracarboxylic dianhydride. The polymer thus activated is then allowed to react with the collagen, forming a collagen-polymer composition of the invention.

CONJUGATES WITH ESTER LINKAGES

In one embodiment, a pharmaceutically pure form of monomethylpolyethylene glycol (mPEG) (mw 5,000) is reacted with glutaric anhydride (pure form) to create mPEG glutarate. The glutarate derivative is then reacted with N-hydroxysuccinimide to form a succinimidyl monomethylpolyethylene glycol glutarate. The succinimidyl ester (mPEG*, denoting the activated PEG intermediate) is then capable of reacting with free amino groups present on collagen (lysine residues) to form a collagen-PEG conjugate of the invention wherein one end of the PEG molecule is free or nonbound. Other polymers may be substituted for the monomethyl PEG, as described above. Similarly, the coupling reaction may be carried out using any known method for derivatizing proteins and synthetic polymers. The number of available lysines conjugated may vary from a single residue to 100% of the lysines, preferably 10%-50%, and more preferably 20-30%. The number of reactive lysine residues may be determined by standard methods, for example by reaction with TNBS.

The resulting product is a smooth, pliable, rubbery mass having a shiny appearance. It may be wetted, but is not water-soluble. It may be formulated as a suspension at any convenient concentration, preferably about 30-65 mg/mL, and may be implanted by injection through a suitable syringe. The consistency of the formulation may be adjusted by varying the amount of liquid used.

COMPOSITIONS FOR BONE REPAIR

Formulations suitable for repair of bone defects or nonunions may be prepared by providing high concentration compositions of biocompatible conjugate, or by admixture with suitable particulate materials. When making bone repair compositions, the linkage between the collagen and polymer must be an ether linkage in order to avoid deterioration due to the hydrolysis of the ester linkages. Such conjugate/particulate compositions may be malleable or rigid, depending on the amount of liquid incorporated. Formulations for treatment of stressbearing bone are preferably dried and rigid, and will generally comprise between about 45% and 85% particulate mineral, for example hydroxyapatite or tricalcium phosphate, or mixtures thereof. The tensile strength and rigidity may be further increased by heating the composition under vacuum at about 60°-90° C., preferably about 75° C., for about 5 to 15 hours, preferably about 10 hours. Malleable compositions may be used for repair of non-stressed bone or cartilage.

The activated mPEG* may be replaced, in whole or in part, by difunctional activated PEG (dPEG*, e.g., non-methylated PEG which is then activated at each end), thus providing a crosslinked or partially crosslinked composition. Such compositions are, however, quite distinct from conventionally crosslinked collagen compositions (e.g., using heat, radiation, glutaraldehyde, and the like), as the long-chain synthetic hydrophilic polymer imparts a substantial hydrophilic character to the composition. In a presently preferred embodiment, approximately 1-20% of the mPEG is difunctional PEG. The character of the composition may be adjusted as desired, by varying the amount of difunctional PEG included during the process.

In another presently preferred embodiment, difunctional PEG* (substantially 100% at pH 7) is used to crosslink a biocompatible polymer. In one version, CIS (about 3-100 mg/mL, preferably about 10-40 mg/mL) is allowed to react with dPEG* (difunctional PEG activated at each end by addition of an acid anhydride having a leaving group such as succinimide) having a molecular weight of about 2,000 to about 20,000 (preferably about 3,400-10,000) which is added as a concentrated solution to a final reaction mixture concentration of about 5-40%, preferably about 1-20%. This represents a 5- to 10-fold excess of dPEG* to collagen on a molar basis. The collagen molecules bind to dPEG*, without mechanical mixing or agitation, and settle out of solution to produce a cartilaginoid collagen-polymer conjugate containing approximately 20-80% fibrillar collagen. The conjugate is then washed with PBS to remove any remaining unreacted dPEG*, providing the material of the invention. A cartilaginoid collagen-polymer conjugate may also be prepared by mixing dPEG* solution (pH 3) with collagen-in-solution between two syringes to homogeneity, and then casting into a suitable container (e.g., a Petri dish). A 20% w/v dPEG* solution (pH 7) is then added to the nonfibrillar collagen-PEG solution to result in a lightly cartilaginoid fibrillar collagen-polymer conjugate. The resulting NFC-F conjugate cartilage contains approximately 1-40% fibrillar collagen. The characteristics of the final product may be adjusted by varying the initial reaction conditions. In general, increased collagen and/or polymer concentrations provide a denser, less porous product. By varying the pH of the collagen solution and the dPEG* solution, compositions may be producing over a wide range of fibrillar content. If desired, the denser formulations may be cast or molded into any shape desired, for example into sheets or membranes, into tubes or cylinders, into cords or ropes, and the like.

BREAST IMPLANTS

Collagen-polymer conjugates can also be used as coatings for breast implants. The surface of a standard silicone-shell implant can be chemically derivatized to provide active binding sites for di- or multifunctional PEG-collagen (collagen-PEG-silicone). The presence of the collagen coating bound directly to the silicone via PEG will serve to reduce scar tissue formation and capsular contracture. Unlike typical coated breast implants, scar tissue will not be able to grow between the collagen coating and the surface of the implant itself.

Alternatively, PEG-collagen can be formed into a hollow sphere for use as a breast implant shell. The shell can then be filled with a radiolucent material, such as triglycerides, to facilitate mammography.

COATED MEDICAL DEVICES

The injectable formulations (gels or solutions) may be used to coat implants, catheters, tubes (e.g., for blood vessel replacement), meshes (e.g., for tissue reinforcement) and the like. PEG-collagen formulations can also be used to coat platinum wires, which can then be administered to the site of an aneurysm via catheter. Gels may be prepared with various polymer concentrations and different reaction times. CIS is the preferred starting material where the desired properties are high density, rigidity, viscosity, and translucence. However, one may substitute fibrillar collagen (preferably atelopeptide fibrillar collagen such as ZCI),and obtain products which are more opaque, more flexible, and more susceptible to colonization by cells after implantation. CIS-based materials are presently preferred for coating articles to be implanted, such as catheters and stress-bearing bone implants.

CONJUGATE AND CYTOKINES

Compositions of the invention containing biologically active cytokines such as EGF and TGF-β are prepared by mixing an appropriate amount of the cytokine into the composition, or by incorporating the cytokine into the collagen prior to treatment with activated PEG. By employing an appropriate amount of difunctional PEG, a degree of crosslinking may be established, along with molecules consisting of collagen linked to a cytokine by a synthetic hydrophilic polymer. Preferably, the cytokine is first reacted with a molar excess of dPEG* in a dilute solution over a 3 to 4 hour period. The cytokine is preferably provided at a concentration of about 1 μg/mL to about 5 mg/mL, while the dPEG* is preferably added to a final concentration providing a 30 to 50-fold molar excess. The resulting conjugated cytokine is then added to an aqueous collagen mixture (about 1 to about 60 mg/mL) at pH 7-8 and allowed to react further. The resulting composition is allowed to stand overnight at ambient temperature. The pellet is collected by centrifugation, and is washed with PBS by vigorous vortexing in order to remove non-bound cytokine.

MEMBRANOUS FORMS

Flexible sheets or membranous-forms of the collagen-polymer conjugate may be prepared by methods known in the art, for example., U.S. Pat. Nos. 4,600,533; 4,412,947; and 4,242,291. Briefly, high concentration (10-100 mg/mL) CIS or fibrillar collagen (preferably atelopeptide fibrillar collagen, such as ZCI) is cast into a flat sheet container. A solution of mPEG* (having a molecular weight of approximately 5,000) is added to the cast collagen solution, and allowed to react overnight at room temperature. The resulting collagen polymer conjugate is removed from the reaction solution using a sterile spatula or the like, and washed with PBS to remove excess unreacted mPEG*.

The resulting conjugate may then be compressed under constant pressure to form a uniform, flat sheet or mat, which is then dried to form a membranous implant of the invention. More flexible membranous forms are achieved by using lower collagen concentrations and high polymer concentrations as starting materials.

Less flexible membranous forms are prepared by using a dPEG* solution rather than mPEG*. CIS, at room temperature, is mixed with a buffer solution and incubated at 37° C. overnight. The resulting gel is compressed under constant pressure, dried, and desalted by washing. The resultant membrane is then crosslinked by treating with dPEG*, washed, and then dried at low temperature.

Alternatively, CIS or fibrillar collagen (10-100 mg/mL) is cast into a flat sheet container. A solution of dPEG* (22-50% w/v) is added to the cast collagen. The mixture is allowed to react over several hours at room temperature. Shorter reaction times result in more flexible membranes. The resulting collagen-polymer membrane may be optionally dehydrated under a vacuum oven, lyophilization, or air-drying.

SPONGES

Biocompatible conjugates may also be prepared in the form of sponges, by lyophilizing an aqueous slurry of the composition after conjugation.

B. 2 USE AND ADMINISTRATION compositions of the invention have a variety of uses. Malleable, plastic compositions may be prepared as injectable formulations, and are suitable for dermal augmentation, for example for filling in dermal creases, and providing support for skin surfaces. Such compositions are also useful for augmenting sphincter tissue, (e.g., for restoration of continence). In such cases, the formulation may be injected directly into the sphincter tissue to increase bulk and permit the occluding tissues to meet more easily and efficiently. These compositions may be homogeneous, or may be prepared as suspensions of small microgel collagen-polymer conjugate particles or beads which are delivered in a nonaqueous carrier. The beads/particles rehydrate and swell in situ. This has the advantage over commercial preparations in that less volume of product needs to be used to achieve the desired connection.

Surprisingly, one may administer the reaction mixture by injection before crosslinking has completed. In this embodiment, an aqueous collagen mixture is combined with a low-concentration dPEG* solution, mixed, and the combination injected or applied before the viscosity increases sufficiently to render injection difficult (usually about 20 minutes). Mixing may be accomplished by passing the mixture between two syringes equipped with Luer lock hubs, or through a single syringe having dual compartments (e.g., double barrel). The composition crosslinks in situ, and may additionally crosslink to the endogenous tissue, anchoring the implant in place. In this method, one can use collagen (preferably fibrillar collagen) at a concentration of about 10-100 mg/mL, although abut 30-80 mg/mL is preferred, most preferably about 33 mg/mL. The dPEG* concentration is preferably set at about 0.1 to about 3%, although concentrations as high as 30% may be used if desired. The mixture is injected directly into the site in need of augmentation, and causes essentially no detectable inflammation or foreign body reaction. One may additionally include particulate materials in the collagen reaction mixture, for example hydrogel or collagen-dPEG beads, or hydroxyapatite/tricalcium phosphate particles, to provide a bulkier or more rigid implant after crosslinking.

Compositions of the invention (particularly crosslinked collagen compositions) are also useful for coating articles for implantation or relatively long term residence within the body. Such surface treatment renders the object non-immunogenic, and reduces the incidence of foreign body reactions. Accordingly, one can apply compositions of the invention to catheters, cannulas, bone prostheses, cartilage replacement, breast implants, minipumps, and other drug delivery devices, artificial organs, and the like. Application may be accomplished by dipping the object into the reaction mixture while crosslinking is occurring, and allowing the adherent viscous coating to dry. One may pour or otherwise apply the reaction mixture if dipping is not convenient. Alternatively, one may use flexible sheets or membranous forms of collagen-polymer conjugate to wrap the object with, sealing corners and edges with reaction mixture.

In another embodiment, the object may be dipped in a viscous collagen-in-solution bath, or in a fibrillar collagen solution until the object is completely coated. The collagen solution is fixed to the object by dipping the collagen-coated object into a dPEG* (pH-7) solution bath, and then allowing the collagen-polymer coated object to dry. Alternatively, viscous collagen-in-solution is mixed with a dPEG* (pH 3) solution and polymerized rapidly, as described above. The object is dipped in the acidic collagen-polymer solution, and cured by dipping the coated object into a-neutralizing buffer containing about 20% by weight dPEG* (pH 7), to result in a collagen-polymer coated object.

Compositions of the invention may be prepared in a form that is dense and rigid enough to substitute for cartilage. These compositions are useful for repairing and supporting tissue which require some degree of structure, for example in reconstruction of the nose, ear, knee, larynx, tracheal rings, and joint surfaces. One can also replace tendon, ligament and blood vessel tissue using appropriately formed cartilaginoid material. In these applications, the material is generally cast or molded into shape: in the case of tendons and ligaments, it may be preferable to form filaments for weaving into cords or ropes. In the case of artificial blood vessels it may be advantageous to incorporate a reinforcing mesh (e.g., nylon or the like).

Compositions of the invention containing cytokines are particularly suited for sustained administration of cytokines, as in the case of wound healing promotion. Osteoinductive factors and cofactors (including TGF-β) and bone morphogenic protein (BMP) may advantageously be incorporated into compositions destined for bone replacement, augmentation, and/or defect repair. Compositions provided in the form of a membrane may be used to wrap or coat transplanted organs, to suppress rejection and induce improved tissue growth. Similarly, one may coat organs for transplantation using a crosslinking reaction mixture of factor-polymer conjugates and collagen. Alternatively, one may administer antiviral and antitumor factors such as TNF, interferons, CSFs, TGF-β, and the like for their pharmaceutical activities. The amount of composition used will depend upon the severity of the condition being treated, the amount of factor incorporated in the composition, the rate of delivery desired, and the like. However, these parameters may easily be determined by routine experimentation, for example by preparing a model composition following the examples below, and assaying the release rate in a suitable animal model.

EXAMPLES

The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to make the conjugates and formulations and implants containing such conjugates and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers used (e.g. amounts, temperature, molecular weight, etc.) but some experimental errors and deviation should be accounted for. Unless indicated otherwise, parts are parts by weight, molecular weight is weight average molecular weight, temperature is in degrees Centigrade, and pressure is at or near atmospheric.

EXAMPLE 1 Preparation of Collagen-PEG

(A) Monomethyl-PEG 5000 (50 g, 10 mmol, Aldrich Chemical Co.) is dissolved in 1,2-dichoroethane (250 mL) and heated at reflux with glutaric anhydride (5 g) and pyridine (4 mL) under nitrogen for 3 days. The solution is then filtered and the solvent evaporated, and the residue dissolved in water (100 mL) and washed with diethyl ether (2×50 mL). The resulting PEG-glutarate is extracted from the water with chloroform (2×50 mL), and the chloroform evaporated to yield about 43 g of PEG-glutarate. The PEG-glutarate is then dissolved in dimethylformamide (DMF, 200 mL) at 37° C., and N-hydroxysuccinimide (10% molar xs) added. The solution is cooled to 0° C., and an equivalent amount of dicyclohexylcarbodiimide added in DMF solution (10 mL). The mixture is left at room temperature for 24 hours, and then filtered. Cold benzene (100 mL) is then added, and the PEG-succinimidyl glutarate (PEG-SG) precipitated by adding petroleum ether (200 mL) at 0° C. The precipitate is collected on a sintered glass filter. Dissolution in benzene, followed by precipitation with petroleum ether is repeated three times to provide "activated" PEG (PEG-SG).

Vitrogen 100® collagen in solution (400 mL, 1.2 g collagen, 0.004 mmol) was mixed with 0.2M phosphate buffer (44 mL) to elevate the pH to 7.4. Next, a threefold molar excess of PEG-SG (6.00 g, 1.2 mmol) was dissolved in water for injection (40 mL) and sterilefiltered. The PEG-SG solution was then added to the collagen solution, and the mixture allowed to stand at 17°-22 C. for about 15 hours. The solution was then centrifuged, and the resulting pellet (25 g) of reconstituted fibrils collected and washed with phosphate-buffered saline (PBS, 3X 400 mL) to remove residual PEG. The resulting material has a solid, coherent elasticity, and may be picked up on a spatula (the equivalent nonconjugated collagen, Zyderm® collagen implant is more fluid). The resulting material may be diluted with PBS to provide a dispersion having 20.5 mg/mL collagen-PEG.

(B) Similarly, proceeding as in part (A) above but substituting polypropylene glycol and POE-POP block polymers for polyethylene glycol, the corresponding collagen-PPG and collagen-POE-POP compositions are prepared.

(C) Difunctional PEG 3400 (34 g, 10 mmol, Aldrich Chemical Co.) is dissolved in 1,2-dichoroethane (250 mL) and heated at reflux with glutaric anhydride (10 g) and pyridine (4 mL) under nitrogen for 3 days. The solution is then filtered and the solvent evaporated, and the residue dissolved in water (100 mL) and washed with diethyl ether (2X 50 mL). The resulting PEG-diglutarate is extracted from the water with chloroform (2X 50 mL), and the chloroform evaporated to yield PEG-diglutarate. The PEG-diglutarate is then dissolved in DMF (200 mL) at 37° C., and N-hydroxysuccinimide (10% molar xs) added. The solution is cooled to 0° C., and an equivalent amount of dicyclohexylcarbodiimide added in DMF solution (10 mL). The mixture is left at room temperature for 24 hours, and then filtered. Cold benzene (100 mL) is then added, and the PEG-di(succinimidyl glutarate) (dPEG-SG) precipitated by adding petroleum ether (200 mL) at 0° C. The precipitate is collected on a sintered glass filter. Dissolution in benzene, followed by precipitation with petroleum ether is repeated three times to provide "activated" dPEG (dPEG*).

Vitrogen 100® collagen in solution (400 mL, 1.2 g collagen, 0.004 mmol) was mixed with 0.2M phosphate buffer (44 mL) to elevate the pH to 7.4. Next, a threefold molar excess of dPEG* (6.00 g, 1.2 mmol) was dissolved in water for injection (40 mL) and sterilefiltered. filtered. The dPEG* solution was then added to the collagen solution, agitated, and the mixture allowed to stand at 17°-22° C. for about 15 hours. The solution was then centrifuged, and the resulting pellet of reconstituted fibrils collected and washed with PBS (3X 400 mL) to remove residual dPEG*. The pellet was then placed in a syringe fitted with a Luer lock hub connected to a second syringe, and was passed between the syringes until homogeneous. The resulting material is a microgel or a particulate suspension of random size fibrils in solution (microgel conjugate). The material is a smooth, pliable, rubbery mass, with a shiny appearance.

(D) Preparation of Cartilaginoid Conjugates: Approximately 20% by weight of dPEG* (pH 7) was added to collagen in solution (33.8 mg/mL), and incubated at 21° C. for about 16 hours. The resulting conjugate was washed with 100 mL PBS 3-5 times over 12 hours. The resulting cartilaginoid non-fibrillar collagen-polymer conjugate (NFC-F cartilage) was a translucent solid with coherent elasticity. The product contained approximately 20-80% fibrillar collagen.

Another NFC cartilage composition was prepared by mixing dPEG* solution (0.6 g, pH 3) with collagen in solution (33.8 mg/mL, pH 2). The mixture was passed between two syringes joined by a Luer lock connector to form a homogenous solution. A solution of dPEG* (20% w/v) in a neutralizing buffer was then added to result in a substantially non-fibrillar collagen (NFC) cartilage material. The resulting product contained approximately 1-40% fibrillar collagen.

Alternatively, fibrillar collagen may be used instead of CIS to produce a cartilaginoid fibrillar collagen-polymer conjugate (F cartilage) having an opaque appearance and high fibrillar content. Such F cartilage is more porous and permeable than non-fibrillar biocompatible conjugates.

EXAMPLE 2 Characterization

(A) Collagen-mPEG prepared in Example 1A was characterized and compared with Zyderm® collagen implant (ZCI), and glutaraldehyde-crosslinked fibrillar collagen (GAX).

EXTRUSION

This assay measured the force required to extrude the test composition through a 30 gauge needle. The results are shown in FIG. 1. As can be seen from the graph of force required (in Newtons) versus plunger travel, ZCI was extruded smoothly, requiring a force of about 20-30 Newtons. GAX was not extruded smoothly, as shown by the "spiking" exhibited in the force trace. At the plateau, GAX required about 10-15N for extrusion. In contrast, collagen-mPEG demonstrated a very low extrusion force (8-10N), with little or no spiking.

INTRUSION

Intrusion is a measure of the tendency of a composition to "finger" or channel into a porous bed, rather than remaining in a compact mass. Low intrusion is preferred in augmentation of soft tissue, so that the injected implant does not diffuse through the dermis and remains in place.

A 1 mL syringe fitted with a 30 gauge needle was half-filled with silicon carbide particles (60 mesh), Simulating human dermis. The upper half of the syringe was filled with 0.5 mL test composition (GAX, ZCI, or collagen-mPEG) at 35 mg/mL. The plunger was then fitted, and depressed. On depression, ZCI appeared at the needle, demonstrating intrusion through the silicon carbide bed. Syringes filled with GAX or collagen-mPEG of the invention did not pass collagen, instead releasing only buffer, demonstrating no intrudability.

HELICITY

The portion of each composition exhibiting nonhelical character was measured using sensitivity to digestion with trypsin. Samples were treated with the protease trypsin, which is capable of attacking only fragmented portions of the collagen protein. The extent of hydrolysis is measured by fluorescamine assay for solubilized peptides, and the results are expressed as percentage non-helical collagen. The percentage of nonhelical collagen was measured 30 minutes after the beginning of the digestion period. The results indicated that ZCI was 3-10% sensitive, GAX was 1-2% sensitive, and collagen-mPEG was about 1% sensitive. Sensitivity to trypsin may also correlate to sensitivity to endogenous proteases following implantation.

COLLAGENASE SENSITIVITY

The sensitivity of each composition to collagenase was also measured. ZCI was 65.2% digested, compared to 2.2% for GAX, and 45.8% for collagen-mPEG.

PHASE TRANSISTION

The behavior of each composition vs. temperature was examined using a differential scanning calorimeter. On heating, ZCI exhibited multiple peaks at about 45° and 53° C. GAX exhibited a peak at 67°-70° C. Collagen-mPEG exhibited a peak at 56°-61° C.

LYSINE CONTENT

The number of free lysines per mole was determined for each composition using TNBS to quantify reactive epsilon amino groups. ZCI exhibited about 30 lysines per (single helix) molecule (K/m), whereas GAX exhibited 26-27 K/m, and collagen-mPEG 21-26 K/m.

(B) Characterization of Crosslinked Biocompatible conjugates: A collagen-dPEG conjugate prepared as described in Example 1C was characterized using differential scanning calorimetry (DSC). This test is a measure of the transition temperature during fragmentation of the collagen molecule at a microscopic level. A lowering of the transition temperature indicates an increase in fragmentation in a manner similar to that measured by trypsin sensitivity.

The collagen-dPEG conjugate showed a single denaturational transition at 56° C. by DSC, which is similar to the typical melting point of the Collagen-PEG conjugate prepared in Example 1A. In comparison, ZCI has a melting temperature of 45°-53° C. with multiple denaturational transitions, and GAX has a melting temperature of 67°-70° C. with a single denaturational transition.

The extrusion test described in Example 2A could not be used to characterize the collagen-dPEG conjugate because the material was not extrudable through a 30 gauge needle.

Using the intrusion test described in Example 2A, the passage of collagen-dPEG was completely blocked at the silicon carbide bed, which indicates high crosslinking between the collagen molecules and little or no intrudability.

EXAMPLE 3 Immunogenicity

(A) Non-crosslinked PEG-Collagen: This experiment was conducted to demonstrate the relative immunogenicity of a collagen-mPEG preparation of the invention versus a commercially-available bovine collagen formulation prepared from essentially the same source material, and having a similar consistency. As both collagen preparations were prepared using atelopeptide collagen (which is only weakly immunogenic), the preparations were formulated with either complete Freund's adjuvant (CFA) or incomplete Freund's adjuvant (IFA), to enhance the immune response. This is a severe test, designed to magnify any possible immune reaction.

Collagen-mPEG was prepared as in Example 1A above. Male Hartley guinea pigs (11) were anesthetized and bled by heart puncture for pre-immunization serologic evaluation. Five animals were treated with two 0.1 mL intramuscular injections of Zyderm® collagen implant (ZCI) emulsified in CFA (1:9) in the left and right thighs. Another five animals were treated in the same fashion, using collagen-PEG (35 mg/mL) emulsified in CFA. One animal was treated with collagen-PEG in IFA. At day 14 following immunization, all animals were again bled by heart puncture, and serum obtained for antibody titer determination (using ELISA). Serology was again performed at day 30.

On day 30, following collection of serum samples, each animal was challenged intradermally with both ZCI and collagen-PEG (0.1 mL of each, one on each flank). Delayed-type hypersensitivity (DTH) was quantified as a measure of cell-mediated immunity. DTH was evaluated at 24, 48, and 72 hours post-challenge by measuring the diameter of any wheal using micrometer calipers, and noting the extent of erythema and induration. Animals were then euthanized with CO₂, and the injection sites excised and fixed in neutral, buffered formalin for histological study.

Serological results indicated reduced immunogenicity of collagen-PEG vs. ZCI. At day 14, 80% of ZCI immunized animals exhibited "positive" antibody responses (titer≧160 at day 14), whereas 0% of the collagen-PEG immunized animals exhibited positive responses. At day 30, all ZCI-immunized animals exhibited high antibody titers, whereas none of the collagen-PEG-immunized animals (C-PEG) exhibited high titers. The data are shown in Table 1.

                  TABLE 1                                                          ______________________________________                                         Immunogenicity                                                                                    Antibody Titer                                              Animal  Treatment        day 14  day 30                                        ______________________________________                                         1       ZCI              320     >2560                                         2       ZCI              320     1280                                          3       ZCI              2560    >2560                                         4       ZCI              320     >2560                                         5       ZCI              80      2560                                          6       C-PEG            0       0                                             7       C-PEG            0       160                                           8       C-PEG            40      640                                           9       C-PEG            0       20                                            10      C-PEG            0       640                                           11      C-PEG (IFA)      0       160                                           ______________________________________                                    

Responses to the DTH challenge also demonstrated that the collagen-mPEG of the invention is less immunogenic. Guinea pigs immunized with ZCI and challenged with ZCI exhibited a wheel measuring 1,128±0.058 cm in diameter. Animals immunized with collagen-mPEG and challenged with collagen-mPEG exhibited wheals measuring 0.768±0,036 cm. Animals immunized with ZCI and challenged with collagen-mPEG, or immunized with collagen-mPEG and challenged with ZCI, developed wheals smaller than the ZCI-immunized ZCI-challenged wheals. Responses measured at 48 and 72 hours were essentially the same or lower than the 24 hour response for each site. Erythema was essentially the same for all animals.

Histological studies showed that both materials exhibited comparable intrusion, fingering into the dermis and subcutaneous space. Sites of intradermal challenge with ZCI in ZCI-immunized animals exhibited the most extensive inflammatory response, including a cellular infiltrate of lymphohistiocytic elements with eosinophils and occasional giant cells. Two of the implant sites demonstrated an erosive inflammation of the overlying epidermis and eschar formation. Sites of intradermal challenge with collagen-mPEG in ZCI-immunized animals exhibited only a moderate associated inflammatory infiltrate, with a marked reduction in acute cells and lymphoid elements. Histiocytes and giant cells were more prevalent, and in some samples lined and colonized the implants heavily. Animals immunized with collagen-mPEG exhibited only slight to moderate reaction, with ZCI challenge sites accompanied by a modest lymphohistiocytic perivascular infiltrate with a few eosinophils and giant cells. Collagen-mPEG challenge sites were typically accompanied by a minimal scattering of lymphoid cells near the associated vasculature.

(B) Crosslinked dPEG-Collagen Conjugates: Collagen-dPEG conjugates were prepared as in Example 1D. The samples were implanted in the dorsal subcutis and as cranial onlays in rats. After implantation for 30 days in the subcutis, NFC cartilage and NFC-F cartilage materials had a homogeneous microfibrillar structure. Mild colonization by connective tissue cells occurred at the periphery of the NFC-F cartilage samples, and mild capsule formation was present. No colonization had occurred with the NFC cartilage material and mild capsule formation was present. F cartilage had a very fibrous structure with mild but frequently deep colonization by connective tissue cells and sparse numbers of adipocytes. Trace amounts of capsule were present in limited areas of the F cartilage samples. NFC cartilage materials tended to retain their pre-implantation shape, with sharply defined edges, while the NFC-F cartilage samples tended to flatten over time and develop rounded profiles.

When implanted as cranial onlays, the appearance of each of the materials was similar to that in the subcutis except that the samples tended to become anchored to the skull via integration of the capsule or surrounding loose connective tissue with the periosteum.

All of the samples appeared to be biocompatible, have differing degrees of colonization by host tissues, and varying mechanical characteristics.

EXAMPLE 4 In situ Crosslinking

A dPEG solution was prepared as described in Example 1C above. The following samples were then prepared:

(1) 5 mg dPEG in 80 μL water, mixed with 0.5 mL fibrillar collagen (35 mg/mL), to a final dPEG concentration of 1% by volume;

(2) 15 mg dPEG in 80 μL water, mixed with 0.5 mL fibrillar collagen (35 mg/mL), to a final dPEG concentration of 3% by volume;

(3) Vitrogen® 100 collagen in solution;

(4) 5 mg dPEG in 80 μL water, mixed with 0.5 mL non-fibrillar collagen (35 mg/mL), to a final dPEG concentration of 1% by volume;

(5) 15 mg dPEG in 80 μL water, mixed with 0.5 mL non-fibrillar collagen (35 mg/mL), to a final dPEG concentration of 3% by volume;

(6) 5 mg dPEG in 0.5 ml PBS, to a final dPEG concentration of 1% by volume; and

(7) GAX.

The dPEG solutions of Samples 1, 2, 4, and 5 were placed in a i mL syringe equipped with a Luer lock fitting and connector, and joined to another syringe containing the collagen material. The solutions were mixed by passing the liquids back and forth between the syringes several times to form the homogeneous reaction mixture.

The syringe connector was then removed and replaced with a 27 gauge needle, and approximately 50 μL of the reaction mixture was injected intradermally into each of 20 guinea pigs. Samples 3, 6, and 7 were similarly administered through a 27 gauge needle. At intervals up to 30 days following injection, the treatment sites were harvested and studied histologically.

By 30 days, all of the materials appeared to be biocompatible. Samples 1 and 2 displayed wide dispersion with an intermediate degree of interdigitation with dermal collagen fibers. Colonization by connective tissue cells was moderate, and a trace of round cell infiltrate with eosinophils was seen.

Samples 3, 4 and 5 were highly dispersed and finely interdigitated with dermal collagen fibers. Colonization was mild to moderate, and trace levels of round cell infiltration were seen.

Sample 6 had no detectable effects. Sample 7 occurred as large islands with moderate colonization and trace to mild levels of inflammation.

EXAMPLE 5 Coating of Implants

A collagen-dPEG reaction mixture was prepared as described in Example 1C above. A titanium implant was dipped into the reaction mixture approximately 20 minutes after crosslinking was initiated. The implant was then allowed to finish crosslinking, and dry overnight.

EXAMPLE 6 Collagen-Polymer-Growth Factor Conjugates

(A) A conjugate containing crosslinked collagen-dPEG-TGF-β1 was prepared as follows:

A solution of TGF-β1 and ¹²⁵ I-TGF-β1 (10⁵ cpm; 25 μL of 1 mg/mL) was added to a solution of dPEG* (4 mg) in CH₂ Cl₂ (100 μL), and the mixture allowed to react for 12 (sample #3) or 35 (sample #5) minutes at 17° C. To this was added 2.5 mL of collagen solution (3 mg/mL atelopeptide nonfibrillar collagen), and the resulting mixture allowed to incubate overnight at ambient temperature. The pellet which formed was collected by centrifugation to provide collagen-dPEG-TGF-β1.

(B) A composition based on fibrillar atelopeptide collagen was prepared as in part A above, but limiting TGF-β1 dPEG* reaction time to 2 minutes, and substituting 7 mg of fibrillar collagen (precipitated from collagen in solution within 2 minutes prior to use) for collagen in solution.

(C) A composition containing dPEG-crosslinked collagen and free TGF-β1 was prepared as follows:

A solution of dPEG* (4 mg) in CH₂ Cl₂ (100 μL), was added to 2.5 mL of CIS (3 mg/mL atelopeptide nonfibrillar collagen), and the resulting mixture allowed to incubate overnight at ambient temperature. The pellet which formed was washed to remove unreacted dPEG*, and 25 μg of TGF-β1 mixed in to provide collagen-dPEG +TGF-β1.

(D) The degree of TGF-β1 binding was determined as follows:

Each composition prepared in parts A-C above was washed six times with 0.5 mL of buffer (0.02M phosphate buffer, 0.1% BSA) by vigorous vortexing followed by centrifugation in order to remove non-bound TGF-β1. The pellet and supernatants were collected at each time of washing, and were counted. FIG. 2 demonstrates the release rate of the compositions of part A (open circles) and part B (filled circles) versus the simple mixture prepared in part C (x's), showing the number of counts released as a function wash cycle. As shown in the figure, the TGF-β1 in the simple mixture is quantitatively released within about 6 washings, while approximately 40% of the TGF-β1 is retained in the compositions of part B and 50% is retained in the compositions of part A.

(E) The biological activity of the materials prepared above was assayed as follows:

Compositions prepared according to part A (CISdPEG-TGF-β1) (TGF-β1/dPEG* reaction time of 12 minutes) and part C (CIS-dPEG+TGF-β1) were prepared, as well as a control prepared according to part C without TGF-β1 (CIS-dPEG). The samples were washed in PBS/BSA eight times as described in part D, then washed an additional three times in fetal bovine serum (Gibco) at 37° C. This washing protocol resulted in visually detectable material loss, so remaining TGF-β1 content was determined by counting the remaining ¹²⁵ I. TGF-β1 activity was then assayed by ELISA. The results are shown in Table 2 below.

                  TABLE 2                                                          ______________________________________                                         Retention of Biological Activity                                                             125.sub.I                                                                               remaining   O.D.                                        Sample        Counts   TGF-β1 (μg)                                                                        (414 nm)                                    ______________________________________                                         CIS-dPEG        0      0           0.015                                                                          0.015                                       CIS-dPEG + TGF-β1                                                                        2775    0.5-1.0     0.029                                                                          0.035                                       CIS-dPEG-TGF-β1                                                                         42604    7.4         0.102                                                                          0.082                                       ______________________________________                                    

The data demonstrates that the TGF-β1 retained in the compositions of the invention remains in a substantially active form.

EXAMPLE 7 Formulations

(A) A formulation suitable for implantation by injection was prepared by suspending collagen-PEG in sterile water for injection, at 35 mg/mL. The characteristics of the resulting formulation are described in Example 2 above.

(B) A formulation useful for repair of stress-bearing bone defects (e.g., fractures, nonunions, and the like) may be prepared by mixing collagen-PEG of the invention with a suitable particulate, insoluble component. The insoluble component may be fibrillar crosslinked collagen, gelatin beads, polytetrafluoroethylene beads, silicone rubber beads, hydrogel beads, silicon carbide beads, mineral beads, or glass beads, and is preferably a calcium mineral, for example hydroxyapatite and/or tricalcium phosphate.

Solid formulations were prepared by mixing Zyderm® II (65 mg/mL collagen) or collagen-mPEG (63 mg/mL) with particulate hydroxyapatite and tricalcium phosphate (HA+TCP) and air drying to form a solid block containing 65% HA by weight. Optionally, blocks were heat-treated by heating at 75° C. for 10 hours. The resulting blocks were hydrated in 0.13M saline for 12 hours prior to testing.

On standing, it was observed that Zyderm®-HA+TCP (Z-HA) compositions separated into three phases, whereas PEG-collagen-HA+TCP (PC-HA) compositions remained single phase.

Each block was elongated by 5%, and its stress relaxation monitored for 1 minute after release. After this test, each block was subjected to constant elongation at a constant 1 cm/min until failure. The results are shown in Table 3:

                  TABLE 3                                                          ______________________________________                                         Mechanical Strength                                                            Stress Relaxation     Constant Extension                                              Peak     Constant t.sub.1/2                                                                             Rupture                                                                               Extension                               Sample Force    Force    (min)  Force  at Rupture                              ______________________________________                                         Z-HA   1.5      1.1      0.04   2.6    11.0%                                   (air)  --       --       --     2.6    15.3%                                   Z-HA   1.5      1.1      0.06   --     --                                      (heat) 1.4      1.0      0.07   3.4    14.0%                                   PC-HA  2.6      1.8      0.06   5.5    12.3%                                   (air)  2.8      2.1      0.08   5.4    11.7%                                   PC-HA  3.3      2.6      0.04   5.4    12.0%                                   (heat) 3.6      2.7      0.06   5.4    20.3%                                   ______________________________________                                          All forces reported in newtons. Extension at rupture (strain) reported in      percent extension.                                                       

The data demonstrate that collagen-polymer forms HA+TCP compositions exhibiting substantially greater tensile strength. Thus, one can prepare implant compositions with collagen-polymer which are substantially stronger than compositions employing the same amount of non-conjugated collagen, or may reduce the amount of collagen-polymer employed to form a composition of equal strength.

EXAMPLE 8 Crosslinking of Hyaluronic Acid With Difunctional SC-PEG

One (1) gram of sodium hyaluronate (obtained from LifeCore Biomedical) was added to 15 ml of PBS and allowed to dissolve overnight to form a homogeneous solution. Five (5) ml of the hyaluronic acid/PBS solution was mixed with 50 mg of difunctional SC-PEG in 0.5 ml of PBS using syringe-to-syringe mixing.

The resulting material was extruded from the syringe into a petri dish and incubated at 37° C. for 16 hours. The material was then allowed to cool at room temperature for 8 hours. After 24 hours, the material had formed a crosslinked gel.

Hyaluronic acid without S-PEG was used as a control in this experiment. After the same incubation period, the control was still liquid and runny.

The instant invention is shown and described herein at what is considered to be the most practical, and preferred embodiments. It is recognized, however, that departures may be made therefrom which are within the scope of the invention and that obvious modifications will occur to one skilled in the art upon reading this disclosure. 

What is claimed:
 1. A biocompatible, biologically inactive conjugate comprising an inert, naturally occurring biocompatible polymer or derivative thereof chemically conjugated to a synthetic non-immunogenic hydrophilic polyethylene glycol polymer.
 2. The conjugate of claim 1, wherein the naturally occurring biocompatible polymer is a polysaccharide.
 3. The conjugate of claim 3, wherein the polysaccharide is hyaluronic acid or a derivative thereof.
 4. The conjugate of claim 3, wherein the polysaccharide is cyclodextrin.
 5. The conjugate of claim 3, wherein the polysaccharide is hydroxymethyl cellulose.
 6. The conjugate of claim 3, wherein the polysaccharide is cellulose ether.
 7. The conjugate of claim 3, wherein the polysaccharide is a starch.
 8. The conjugate of claim 1, wherein the synthetic hydrophilic polyethylene glycol polymer is a succinimidyl monomethyl-polyethylene glycol glutarate.
 9. The conjugate of claim 1, wherein the conjugate has the following general structural formula:

    hyaluronic acid--HN--OC--(CH.sub.2).sub.n --O--PEG--O--(CH.sub.2).sub.N --CO--NH--hyaluronic acid

wherein PEG is the synthetic non-immunogenic hydrophilic polyethylene glycol polymer and n is an integer selected from the group consisting of 0, 1, 2, 3, or
 4. 10. The composition of claim 1 wherein the weight ratio of the naturally occurring polymer to the synthetic polyethylene glycol polymer is in the range of about 20:1 to about 1:20.
 11. The conjugate of claim 1, wherein the polyethylene glycol polymer has a molecular weight in the range of about 100 to about 20,000.
 12. The conjugate of claim 11, wherein the molecular weight is in the range of about 1,500 to 20,000.
 13. The conjugate of claim 1, wherein the naturally occurring biocompatible polymer is chemically conjugated to the polyethylene glycol polymer by a chemical linkage selected from their group consisting of an ester bond, a urethane bond and an ether bond.
 14. The conjugate of claim 1, wherein the naturally occurring biocompatible polymer is chemically conjugated to the polyethylene glycol polymer by an ether bond. 